Include InstallNLogConfig.exe in NuGet packages

We should include InstallNLogConfig.exe in the NuGet packages which
support it.
fixes #200
Create NuGet package for snippets

Create a new NuGet package for installing snippets
fixes #151
Add NuGet to the tools directory

Currently we require NuGet to be in PATH when using the build scripts. Not
all users have this. So NuGet.exe is added to the tools directory
Only build Silverlight versions if available

Not all users have all the versions, if any, of Silverlight installed. So
the build script is updated to only build them, if they are available,
unless the project has been specified explicitly.
Fix error in MSBuild file
Add NLog for .NET 4.5
Add support for Silverlight 5

Add Silverlight 5 project.
fixes #203
Update buildversion to 2.0.1
Updated NuGet package info
Remove reference to System.IO.Abstractions
Remove test references to System.IO.Abstraction
Updated file version
Updated NLog.Extended version
Place InstallNLogConfig.exe in tools